The invention relates to a device for optically scanning an information plane, which device comprises a radiation source for supplying a scanning beam, an objective system for focusing the scanning beam to a scanning spot on the information plane, a dividing element comprising at least one sector having an included arc equal to or smaller than 120.degree. arranged in the optical path of the radiation from the information plane for splitting at least a part of said radiation into a sub-beam, and a radiation-sensitive detection system comprising an oblong detector for receiving the sub-beam.
A sector is an area enclosed between two radii and a line connecting endpoints of the radii. The line may have any form, e.g. an arc cut off by the radii or a straight line, or a line having a 90.degree. angle. A bisector of a sector is a straight line which bisects the included angle between the two radii of the sector in equal parts. A figure is said to be oblong when its dimensions in perpendicular directions are not equal.
A device of this type, which in principle is suitable for reading and writing information in an optical record carrier, is known from a publication in the journal Neues aus der Technik, number 4, Dec. 20, 1990, page 7, item 493. The sub-beams formed by a dividing element of the known device are incident on a detection system. The detection signals of the detection system are used for deriving a radial tracking error signals and a focus error signal. It is a disadvantage of the known device that it is less suitable for scanning multi-layer record carriers, as known from inter alia U.S. Pat. No. 5,511,057. The information planes in such a multi-layer record carrier are scanned from one side of the record carrier, i.e. one information plane is scanned through another information plane. Stray light from the other information plane affects the signals derived from the detection system. Moreover, neither is the known device suitable for scanning different types of record carriers having transparent substrate layers of different thicknesses, as is known from inter alia European patent application no. 763 236. Aberrated light rays in this applications affect the signals derived from the detection system.